(1) Field of the Invention
The present invention relates to a generation technique of a terahertz (THz) electromagnetic wave.
(2) Description of the Related Art
Electromagnetic waves which have a frequency ranging from 0.1 THz to 10 THz (in the terahertz region) belong to a boundary between light and electric waves. They have the following characteristics: transmitability which is a characteristic of electric waves; and telecentricity which is a characteristic of light. Also, an electromagnetic wave in such region (called “terahertz wave” hereinafter) has a number of absorption spectrums each of which is unique to a substance. Therefore, it is expected that terahertz waves are used in a number of industrial fields such as medical applications, environmental measurements and engineering applications. They include especially: an inspection of an object in an envelop; a food inspection; an inspection of personal belongings; a drug analysis; a medical checkup for skin cancer; a measurement of the quantity of semiconductor impurities; and a complex dielectric constant evaluation. Thus recent years have seen active research and development about terahertz waves.
A widely used generation method of a terahertz wave is the method using a photoconductive element. A conventional example will be described below with reference to FIG. 1 (refer to Reference Document 1: pp. 7853 to 7859 of “Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs”, Applied Optics, vol. 36, No. 30, (1997), M. Tani et. Al.). A low-temperature-grown GaAs layer 92 is formed on a semi-insulating GaAs substrate 91 using the molecular beam epitaxial method. It is known that the GaAs layer 92 has a picosecond carrier lifetime that is 10−12 sec or less maintaining a comparatively large carrier mobility when the layer is formed under a low temperature. This is why the layer is useful as a film for terahertz waves having a high-speed photoconductiveness. On the surface of the GaAs layer 92 a positive electrode 2 and a negative electrode 3 are formed. Both the positive and negative electrodes constitute a dipole antenna that is easy to radiate terahertz waves. Each of these electrodes is T-shaped, and the smallest gap between these electrodes is 5 μm. A power supply 5 applies approximately 30 V to between the positive electrode 2 and the negative electrode 3. A femtosecond pulse light 8 is radiated from a laser radiation exit 7 to the gap on the GaAs layer 92. The pulse light 8 is an approximately 80 fsec Ti:sapphire laser having a wavelength of approximately 780 nm, the laser having been mode-locked by Argon laser excitation. This generates electrons on the photoconductive film (low-temperature-grown GaAs layer 92). The generated electrons become a picosecond order single pulse current, and flow between both the electrodes. This causes a dipole antenna to radiate terahertz wave 10 toward the direction of the substrate 91. These spectrums of the radiation light ranges from a direct current to a frequency of terahertz, and thus electromagnetic waves having a wide band that covers up to a terahertz wave can be obtained.
In order to make terahertz wave power bigger, it is desirable that the bias voltage between the positive electrode 2 and the negative electrode 3 be high.
However, the Reference Document 2 (Japanese Laid-Open Patent Application NO. 2004-22766 publication) describes the following problem. When high voltage is applied to between both the electrodes for a long time, thermally excited carriers increase the amount of current that flows between both the electrodes resulting in decreasing the generation efficiency of terahertz waves. A countermeasure described in the Reference Document 2 is to prevent temperature increase and to control thermal excitation by using forced cooling.
Also, the Reference Document 3 (Japanese Laid-Open Patent Application No. 4-296430 publication) describes the method for emitting an electron pulse wave by applying a pulse light to a photoelectron emitting plane and emitting terahertz wave using the electron pulse wave. This method is for emitting electromagnetic waves causing Cherenkov radiation by accelerating the electron pulse wave. This method requires a distance that enables accelerating the electron pulse wave up to light speed. This is why it is difficult to realize a terahertz wave generation source that is practical and compact.